In certain technological processes and fabrication procedures, as well as in many every-day situations, it is of crucial importance to utilize objects with strongly water-repellent surfaces that are stable enough to retain the water-repellent property even after water exposure. Various substrate surfaces which are smooth and planar at the molecular level, like mica and glass surfaces, can be rendered hydrophobic by means of well-established methods, such as deposition of a monolayer of lipid molecules or fluorocarbons with polar end groups, or, by means of some specific chemical reaction like treatment with alkylthiol of a thin gold layer that in a prior step has been deposited on the substrate surface. In this way, the contact angle for a droplet of water residing on a smooth substrate surface can be raised to a maximum of about 100-120 degrees.
Early on it was found, however, that one can realize even higher contact angle values, in fact approaching the theoretical maximum of 180 degrees, by employing substrate surfaces that are structured geometrically on a colloidal length scale, i.e. about 10−8-10−5 m. In other words, in this context it is advantageous if the resulting hydrophobic surface possesses an unevenness that magnifies the contact surface between water and the hydrophobic surface to a significant extent. Evidently, this means that the actual contact surface with water is much larger than the projected, macroscopic surface, implying that it becomes thermodynamically unfavourable with complete (homogeneous) wetting in spite of the fact that an interface between water and hydrocarbon per se is characterized by a relatively low free surface energy, about 50 mJ per square meter. As a consequence, a number of thin air pockets exist between the water phase and the hydrophobic surface (heterogeneous wetting). In this situation, an approximately planar water-air interface with a surface tension of about 72 mJ per square meter rests attached to high peaks in the “mountain landscape” representing the hydrophobic surface while the valleys are filled with air (FIG. 1), cf. papers published by Cassie and Baxter (1) and Wenzel (2).
Solid surfaces of the kind discussed that exhibit a contact angle toward pure water in the range between about 150 and 180 degrees are commonly denoted as superhydrophobic surfaces. A well-known example taken from nature itself is the leaf of the lotus plant (Nelumbo nucifera). It is striking how easily a water droplet can move by rolling on a super-hydrophobic surface as soon as there is the slightest deviation from the horizontal plane. The reason for this behaviour is the comparatively weak total adhesion force that binds the droplet to the surface as only completely wetted portions of the solid surface contribute. The similarity in behaviour with a small mercury droplet is obvious though in the latter case the adhesion force becomes small mainly as a result of the high surface tension of the mercury droplet hindering substantial deviations from spherical shape. Furthermore, a superhydrophobic surface is, as a rule, “self-cleaning” which means that particles of dust and dirt which at first adhere to the surface are being transferred to water droplets sprinkled onto the surface and then removed when the droplets roll off the surface.
Onda and coworkers (3) have devised a method for rendering glass and metal surfaces superhydrophobic that is based upon smearing a molten wax (alkylketendimer, AKD) on the substrate surfaces followed by crystallization. Furthermore, a Japanese group of researchers have submitted a patent application based upon forming a superhydrophobic AKD-film on Pt/Pd surfaces and thereby transferring the fractal structure to the Pt/Pb film (4).
Despite previous efforts, there is still a need in the art for improving control and scaling up the application of strongly water-repellent materials and surfaces, in order to facilitate production as well as limiting the material use.
Hence, it is the object of the invention to meet these demands.